If the recent measurement of B-mode polarization by BICEP2 is due to primordial gravitational waves, it implies that inflation was driven by energy densities at the grand unified theory scale $M_{\mathrm{GUT}}\sim 2\times {10}^{16}\mathrm{GeV}$. This favors single-field chaotic inflation models. These models require trans-Planckian excursions of the inflaton, forcing one to address the UV completion of the theory. We use a benchmark 4d effective field theory of axion–four-form inflation to argue that inflation driven by a quadratic potential (with small corrections) is well motivated in the context of high-scale string theory models; that it presents an interesting incitement for string model building; and that the dynamics of the UV completion can have observable consequences.

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